The number of diagonals in a hexagon is :

Given:

A hexagon has n = 6 sides.
Formula used:
Number of diagonals in an  n-sided polygon is 
​$$D = \frac{n(n-3)}{2} \\[6pt]$$​​
Solution:
$$D = \frac{6(6-3)}{2} \\ \ \\= \frac{6 \cdot 3}{2} \\ \ \\= \frac{18}{2} \\= 9 \\[6pt]$$​​
Therefore, the correct option is (a) 9.